1. Field of the Invention
The present invention relates to an injection-molding machine and, particularly, to a nozzle touch mechanism for the injection-molding machine.
2. Description of the Related Art
In a nozzle touch mechanism, which moves an injection unit as a whole to bring it into contact with a metal mold, the injection unit, mounted on a base so as to move backward and forward, is driven by a nozzle touch force generating device to bring the nozzle of the injection unit into contact with the metal mold. A known nozzle touch force generating device utilizes a hydraulic cylinder or a spring force expanded by the force of a motor to hold the nozzle in contact with the metal mold (see Japanese Examined Patent Publication (Kokoku) No. 6-17038)
FIG. 2 illustrates a conventional nozzle touch mechanism using a spring as a nozzle touch force generating unit. FIG. 3 is an explanatory view of a unit base for generating the nozzle touch force. As shown, on a rail 2 provided on the base 1 of the injection-molding machine, there is arranged a unit base 11, that constitutes a portion of the injection unit 10 that can move backward and forward.
A nozzle touch force generating unit is arranged on the unit base 11. The nozzle touch force generating unit is constituted by a motor 3 secured to the base 1, a coupling member 9 secured to the base 1 for coupling the rotor shaft of the motor 3 to a ball screw 4 and for supporting the ball screw 4 maintaining the freedom of rotation but inhibiting its motion in the axial direction, a nut member 5 screwed onto the ball screw 4 and positioned between the legs 11a and 11b of the unit base 11, guide rods 7, 7 for guiding the nut member 5, and springs 6, 6 arranged between the nut member 5 and the leg 11a of the unit base 11 which undergo expansion and contraction accompanying the motion of the nut member 5. As the motor 3 is driven to rotate the ball screw 4, the nut member 5 screwed onto the ball screw 4 moves in the axial direction being guided by the guide rods 7, 7. As the nut member 5 moves forward (toward the left in FIGS. 2 and 3), the spring 6 is contracted, the unit base 11 moves forward, and the injection mechanism mounted on the unit base 11 moves forward. According to this constitution, the injection unit 10 moves forward, a nozzle 19 is brought into contact with a metal mold 22, the amount of expansion or contraction of the spring is detected by a sensor 8, and a nozzle touch force corresponding to the amount of the expansion or contraction is generated.
As described above, in the nozzle touch mechanism using the force of the spring, the nozzle touch force for holding the nozzle in contact with the metal mold is generated by the force of the spring contracted and the amount of contraction of the spring may be detected by a sensor or the like in order to control the nozzle touch force.
However, due to the shock that accompanies the injection, the amount of expansion and contraction of a spring may vary, and the sensor may obtain an erroneous detection. In order to prevent erroneous detection, therefore, there has been known an invention in which a spring that serves as a nozzle touch force accumulating means is attached between a frame and a means that converts a rotational force into a thrust, a buffer member is arranged between an injection unit and the means that converts the rotational force into the thrust, and the shock of injection is absorbed by the buffer member, i.e., an injection unit, an inter-frame buffer mechanism and a spring are arranged in series in a constitution in which the injection unit, the buffer means, a ball screw/nut mechanism (or the means for converting the rotational force into the thrust), the spring and the frame are connected together, so that the spring expands and contracts as the shock of injection is transmitted to the spring, preventing the sensor from erroneously detecting the amount of elongation or contraction of the spring (see Japanese Patent No. 3153800).
As a method not maintaining the nozzle touch force by the spring force or the hydraulic pressure, there has further been known an invention in which a hot runner device for contacting the nozzle is provided with a receiving portion for receiving a wedge block, the injection device is provided with a tilted plate for receiving the wedge block, and after the injection device is driven and moved forward by an air cylinder causing the nozzle to come in contact with the hot runner device, the wedge block is driven by the air cylinder so as to be inserted between the receiving portion and the tilted plate, and the injection device is pushed onto the hot runner device to maintain the nozzle touch force (see Japanese Unexamined Patent Publication (Kokai) No. 10-235680).
Generally, it is desired to maintain the nozzle touch force in the nozzle touch mechanism constant. However, at the time of acceleration of injection, the nozzle touch force for pushing the nozzle touch surface drops by being affected by the force of inertia of a moving part of the injection mechanism. FIGS. 4a and 4b are diagrams illustrating the nozzle touch force in a conventional nozzle touch force generating device utilizing a spring, as shown in FIGS. 2 and 3. FIG. 4a illustrates the injection speed of when an injection screw advances in the injection unit to inject the molten resin in a heating cylinder 18 into a metal mold 22, and FIG. 4b illustrates the nozzle touch force at the time of injection.
When the injection operation starts in a state where a predetermined nozzle touch force fs is generated due to the contraction of the spring 6, there occurs a drop in the nozzle touch force for pushing the nozzle touch surface at the time of acceleration of injection due to the force of inertia of moving parts such as the injection screw and a pusher plate for holding the screw in the injection unit 10. At the time of deceleration of injection, on the other hand, the nozzle touch force increases due to the force of inertia of the moving part of the injection unit 10. FIG. 4b illustrates an increase and a decrease of the nozzle touch force by f1 at the time of acceleration and deceleration of injection. The force, generated during the injection and working to move the injection unit back, is small at the time of low acceleration, but is large at the time of high acceleration.
When the nozzle touch force becomes smaller than a force produced by the resin pressure, the injection unit is pushed back permitting the resin to leak. So far, therefore, the nozzle touch surface had been pushed with a sufficiently large nozzle touch force so that there was no leakage of resin despite the nozzle touch force has dropped being affected by the acceleration of injection. When pushed with a large nozzle touch force, however, the metal mold deflects and/or the parallel accuracy of the mold is not maintained causing problems in regard to a drop in the quality of the molded article and a decrease in the life of the metal mold.